1. Field of the Invention
The present invention relates to an optical recording medium with a large capacity for storing data, capable of recording information therein and reading the information therefrom by the application of light with short wavelength, and an optical recording and reading method using the above-mentioned optical recording medium.
2. Discussion of Background
a. Prior arts concerning a recording medium of a write once read many (WORM) type are as follows:
(1) A recording medium using a cyanine dye as a recording material: disclosed in Japanese Laid-Open Patent Application Nos. 57-82093, 58-56892, 58-112790, 58-114989, 59-85791, 60-83236, 60-89842, and 61-25886.
(2) A recording medium using a phthalocyanine dye as a recording material: disclosed in Japanese Laid-Open Patent Application Nos. 61-150243, 61-177287, 61-154888, 61-24609, 62-39286, 63-37991, and 63-39888.
b. Prior arts concerning a WORM type compact disc (CD-R) are as follows:
(1) CD-R employing the combination of a cyanine dye and a metal reflection layer: disclosed in Japanese Laid-Open Patent Application Nos. 1-159842, 2-42652, 2-13656, and 2-168446.
(2) CD-R employing the combination of a phthalocyanine dye and a metal reflection layer: disclosed in Japanese Laid-Open Patent Application Nos. 1-176585, 3-215466, 4-113886, 4-226390, 5-1272, 5-171052, 5-116456, 5-96860, 5-139044, and 5-139044.
(3) CD-R employing the combination of an azo metal chelate dye and a metal reflection layer: disclosed in Japanese Laid-Open Patent Application Nos. 4-46186, 4-141489, 4-361088, 5-279580, 7-51673, 7-161069, 7-37272, 7-71867, 8-231866, and 8-295811.
c. Prior arts concerning a digital video disk (DVD-R) with a large capacity are as follows:
(1) DVD-R employing the combination of a cyanine dye and a metal reflection layer: reported in PIONEER RandD vol. 6, No. 2 (1996), xe2x80x9cDevelopment of DVD-Recordablexe2x80x9d and xe2x80x9cFundamental Development of DVD-Rxe2x80x9d; ISOM/ODS ""96 (1996), xe2x80x9cHigh Density of Recording on Dye Material Disc Approach for 4.7Gxe2x80x9d; and Japanese Laid-Open Patent Application No. 10-235999.
(2) DVD-R employing the combination of an azomethine dye and a metal reflection layer: disclosed in Japanese Laid-Open Patent Application Nos. 8-198872, 8-209012, 8-283263, and 10-273484.
(3) DVD-R employing the combination of an azo metal chelate dye and a metal reflection layer: disclosed in Japanese Patent Publication No. 5-67438 and Japanese Laid-Open Patent Application Nos. 7-161069, 8-156408, 8-231866, 8-332772, 9-58123, 9-175031, 9-193545, 9-274732, 9-277703, 10-6644, 10-6650, 10-6651, 10-36693, 10-44606, 10-58828, 10-86519, 10-149584, 10-157293, 10-157300, 10-157301, 10-157302, 10-181199, 10-181201, 10-181203, 10-181206, 10-188340, 10-188341, 10-188358, 10-208303, 10-214423, 10-228671, 11-12483, 11-28865, 11-42858, 11-134708, and 11-208111.
(4) DVD-R employing the combination of other dyes than those mentioned above and a metal reflection layer: disclosed in Japanese Laid-Open Patent Application Nos. 10-86517, 10-93788, 10-226172, 10-244752, 10-287819, 10-297103, 10-309871, and 10-309872.
(5) DVD-R employing the combination of an azo metal chelate dye, other dyes, and a metal reflection layer: disclosed in Japanese Patent Publication No. 7-51682 and Japanese Laid-Open Patent Application Nos. 11-34499, 11-48612, and 11-105424.
The DVD-R has been developed as a next generation disc with a large capacity. To increase the recording capacity of the DVD-R, various recording materials that can minimize the size of a recording pit are developed, image compression techniques such as Moving Picture Experts Group-2 (MPEG 2) are introduced, and technical development is conducted to shorten the wavelength of the semiconductor laser used to read the recording pit.
An AlGaInP laser diode with a wavelength of 670 nm is only one red semiconductor laser that is on the market for a bar code reader and a measuring instrument. In line with the trend toward high-density optical discs, the red semiconductor laser has been introduced into the optical storage market. To standardize a driving system for a DVD, laser diodes having wavelengths of 630 to 670 nm are currently employed for the light sources. On the other hand, driving systems for a read-only DVD-ROM are put on the market, with the light sources for the driving systems having a wavelength of about 650 nm.
Under such circumstances, the most preferable medium is a DVD-R that can record information therein and read the information therefrom by the application of light with wavelengths of 630 to 670 nm. However, no recording material has been developed that can exhibit sufficient light resistance and storage stability and that can realize the recording and reading operation in practice when driven by an optical pickup system using a laser beam with a wavelength of 670 nm or less.
A squarylium dye is one of the recording materials that can be used in the recording medium for recording information therein and reading the information therefrom by the application of light with wavelengths of 630 to 670 nm. However, the stability of the squarylium dye is extremely poor because of its noticeable photodeterioration. In contrast to this, an azo metal chelate dye has high light resistance. However, the recording medium shows low reflectance when the azo metal chelate dye is used as a recording material in the recording layer.
Accordingly, it is a first object of the present invention to provide an optical recording medium which is suitable for a DVD-R system using semiconductor laser beams with shorter wavelengths, and shows excellent light resistance and storage stability, in particular, even when the recording medium employs a squarylium dye.
A second object of the present invention is to provide a recording and reading method using the above-mentioned optical recording medium.
The above-mentioned first object of the present invention can be achieved by an optical recording medium comprising a substrate and a recording layer formed thereon, the recording layer comprising at least one squarylium compound and one azo metal chelate compound comprising an azo moiety of formula (I-1) and a metal: 
wherein A and B are each independently a residue for forming an aromatic ring which may have a substituent or a heterocyclic ring which may have a substituent in combination with one or two carbon atoms bounded thereto; and X represents a substituent having an active hydrogen group.
Alternatively, the first object of the present invention can also be achieved by an optical recording medium comprising a substrate and a recording layer formed thereon, the recording layer comprising at least one squarylium compound and one azo metal chelate compound comprising an azo moiety of formula (I-2) and a metal: 
wherein Axe2x80x2 is a residue for forming a heterocyclic ring in combination with a carbon atom and a nitrogen atom bonded thereto; B is a residue for forming an aromatic ring or a heterocyclic ring in combination with two carbon atoms bonded thereto; and X represents a substituent having an active hydrogen group.
The above-mentioned second object of the present invention can be achieved by an optical recording and reading method comprising the steps of recording information in the above-mentioned optical recording medium by applying thereto light with wavelengths of 600 to 720 nm, and reading the information from the recording medium by applying thereto light with wavelengths of 600 to 720 nm.